1. Field of the Invention
The present invention relates to a semiconductor integrated circuit including a high-frequency circuit.
2. Description of the Related Art
In a semiconductor integrated circuit, because of good compatibility with a wiring process, a microstrip line is preferably used as a means which transmits a high-frequency signal.
The microstrip line has a structure in which a signal line, a dielectric layer, and a ground layer are laminated in the order named.
As a conventional ground layer, one metal thin film formed on a substrate is used. In order to obtain an excellent high-frequency characteristic for the microstrip line, the metal thin film which is flat and large as much as possible is desired.
However, in general, a metal thin film formed on a substrate of a semiconductor integrated circuit is very thin. When the area of the metal thin film is increased, cracks or peeling may be frequently caused by mechanical stress.
Therefore, in a conventional technique, some openings to get mechanical stress away from the mechanical thin film are formed in the metal thin film. In order to get the stress away, the openings are desirably arranged as densely as possible. Therefore, the openings are generally arranged in a zigzag pattern.
[Patent Document 1] Japanese Patent Application Laid-Open No. 4-291803
In this case, when the openings are arranged in a zigzag pattern, because of the arrangement shape, signal lines are very difficult to be arranged to avoid positions immediately above the openings. Therefore, the signal lines are arranged immediately above several openings.
When, in particular, a high-frequency signal electric current flows in the signal lines, an electric current is inducted from the grounded metal thin film, and the induced current flows in the metal thin film along the signal lines. When the induced current reaches an opening, the current flows along the circumference of the opening to increase a signal loss disadvantageously.
Since the metal thin film is not formed under the signal lines near the opening, an impedance becomes discontinuous, the signal lines have deteriorated characteristics to, especially, a high-frequency signal.
With respect to this problem, the arrangement of the openings may be changed to prevent the openings from being arranged along the signal lines. However, this breaks the regularity of the arrangement of the openings, and the number of operation processes in a design phase of a semiconductor integrated circuit considerably increases. Depending on circumstances, it is impossible to make such a change in a design rule.